Current trends in so called “green” computing have been of increasing interest in power management for various computing contexts, particularly in large, concentrated installations such as data centers. Indeed, energy consumption in data centers has recently come under scrutiny of the Environmental Protection Agency, which, in 2007, cited data centers as consuming approximately 1.5 percent of total U.S. electrical energy consumption at a cost of roughly $4.5 billion. Accordingly, continuing efforts are being made to reduce energy consumption wherever possible, including by power management at the chip level on equipment installed in data centers.
A power state, as used herein, is an operational state of a system in which electrical energy is consumed at a rate that is at or below a corresponding threshold. A diminished power state can be set by limiting system operation, such as by selectively powering down circuitry, and by suspending or even terminating executing processes. In many cases, a power state is defined by the manner in which energy consumption is reduced, i.e., by the circuits that are powered down, the processes that are terminated, the contents that are retained in memory, etc.
Power states typically transition in a sequence from a shallowest state in which a small number of systems are suspended, to a deepest state where all system processes have been terminated to the greatest extent possible while still allowing the system to be restarted without a fresh reboot. Deeper diminished power states conserve more energy than do shallower states, but the system requires a greater amount of time to recover to its fully operational state. Conventional wisdom has the power states transitioning based on system disuse, whereby as the system remains inactive for longer periods of time, it is transitioned into sequentially deeper diminished power states.